fs: add do_linkat() helper and ksys_link() wrapper; remove in-kernel calls to syscall

Using the fs-internal do_linkat() helper allows us to get rid of
fs-internal calls to the sys_linkat() syscall.

Introducing the ksys_link() wrapper allows us to avoid the in-kernel
calls to sys_link() syscall. The ksys_ prefix denotes that this function
is meant as a drop-in replacement for the syscall. In particular, it uses
the same calling convention as sys_link().

In the near future, the only fs-external user of ksys_link() should be
converted to use vfs_link() instead.

This patch is part of a series which removes in-kernel calls to syscalls.
On this basis, the syscall entry path can be streamlined. For details, see
http://lkml.kernel.org/r/20180325162527.GA17492@light.dominikbrodowski.net

Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net>

1 files changed, 9 insertions, 0 deletions

diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
index 0b4fd684f0f1..827ed917630c 100644
--- a/include/linux/syscalls.h
+++ b/include/linux/syscalls.h
@@ -997,4 +997,13 @@ static inline long ksys_mknod(const char __user *filename, umode_t mode,
 	return do_mknodat(AT_FDCWD, filename, mode, dev);
 }
 
+extern int do_linkat(int olddfd, const char __user *oldname, int newdfd,
+		     const char __user *newname, int flags);
+
+static inline long ksys_link(const char __user *oldname,
+			     const char __user *newname)
+{
+	return do_linkat(AT_FDCWD, oldname, AT_FDCWD, newname, 0);
+}
+
 #endif